1. Field of the Invention
The present invention relates to a method of mining potassium chloride from a subterranean deposit, and more particularly, but not by way of limitation, to an improved method of solution mining potassium chloride from subterranean deposits containing potassium chloride and sodium chloride.
2. Description of the Prior Art
Potassium chloride has been produced commercially for some time by dry mining subterranean mineral deposits which contain relatively high quantities of potassium chloride, e.g., sylvinite deposits. Sylvinite deposits are found in many places, generally at depths of from about 300 to 3,000 feet, and contain sodium chloride as well as potassium chloride.
Commercial dry mining of sylvinite deposits for potassium chloride is similar to the shaft mining of coal in that solid sylvinite ore is brought to the surface through shafts, etc., and then treated to separate potassium chloride from other ore components, primarily sodium chloride. Such dry mining is generally limited to depths above about 3,500 feet because of the high cost of sinking shafts and maintaining sufficient support in mined subsurface rooms.
In some areas, sylvinite deposits rich in potassium chloride are found at depths of from about 4,000 to about 6,000 feet. Since dry mining these sylvinite deposits is prohibited by economic and safety considerations, solution mining of the deposits has heretofore been proposed and utilized. That is, an aqueous medium such as water is pumped through the subterranean ore deposits so that both potassium chloride and sodium chloride are dissolved therein. The brine solution formed is removed from the formation and treated to recover relatively pure potassium chloride. The simplest treatment of potassium chloride-sodium chloride brine for potassium chloride recovery is based on the difference in solubility of potassium chloride and sodium chloride in water at various temperatures. That is, the higher the temperature of a potassium chloride-sodium chloride brine solution, the greater the solubility of potassium chloride, but the lower the solubility of sodium chloride. Thus, upon heating the brine solution, sodium chloride is precipitated, and upon cooling the brine solution, potassium chloride is precipitated, provided the saturation limits of the brine solution are reached.
In the solution mining of subterranean ore deposits containing both potassium chloride and sodium chloride wherein the potassium chloride is present in the ore in a minor quantity as compared to the sodium chloride, it is not possible to obtain a brine saturated in potassium chloride by dissolution of the ore in an aqueous medium. That is, the salts contained in the subterranean deposit dissolve in the aqueous medium in proportion to their presence in the ore until saturation in the most abundant salt is achieved. Thus, in solution mining a subterranean deposit such as sylvinite which is lean in potassium chloride (less than 50% potassium chloride), the solution reaches saturation in sodium chloride with the potassium chloride content considerably less than that required to saturate the solution. In order to recover a practical quantity of potassium chloride from a potassium chloride-sodium chloride brine by cooling the brine, the initial solution must be saturated or at least 90% saturated in potassium chloride.
Heretofore, it has been proposed to concentrate the brine solution obtained from mining subterranean ore deposits lean in potassium chloride by removing water therefrom so that the brine is saturated in potassium chloride. While a variety of methods of removing water have been proposed, conventional evaporation is the most practical method of removing water from a potassium chloride-sodium chloride brine. However, as is well understood by those skilled in the art, evaporation of potassium chloride-sodium chloride brine to reach potassium chloride saturation is an expensive process, and where the brine produced is relatively lean in potassium chloride thereby requiring a considerable quantity of water to be removed, solution mining of the potassium chloride deposit is economically unattractive.
By the present invention, an improved method of solution mining subterranean ore deposits containing sodium chloride and potassium chloride, particularly those deposits lean in potassium chloride as compared to sodium chloride, is provided wherein the water removal required in order to reach saturation in potassium chloride is significantly reduced.